1. Field of the Invention
The present invention relates to an image heating apparatus suitable for use as an image heat fixing apparatus (fixer) mounted to an image forming apparatus such as an electrophotographic copying machine or an electrophotographic printer, and to a heater suitably used for the image heating apparatus.
2. Description of the Related Art
As an image heat fixing apparatus (fixer) to be mounted to an image forming apparatus such as an electrophotographic copying machine or a printer, there exists a film heating type apparatus. The film heating type fixing apparatus includes a heater having an electric heat generation member on a substrate made of a ceramic, a fixing film which moves while being in contact with the heater, and a pressure roller which forms a nip portion together with the heater through the fixing film. Japanese Patent Application Laid-Open Nos. S63-313182 and H04-044075 describe this type of fixing apparatus. A recording material bearing an unfixed toner image is heated while being pinched and conveyed at the nip portion of the fixing apparatus. As a result, the toner image formed on the recording material is fixed onto the recording material by heating. This fixer has an advantage in a short time required for raising temperature to a fixable temperature after starting energizing the heater. Therefore, a printer to which the fixer is mounted can reduce a “first printout time (FPOT)” corresponding to a time length required for outputting a first image after input of a print command. This type of fixer has another advantage in its low power consumption during a standby time for a print command.
Now, it is known that if a printer equipped with a fixer using a fixing film prints small size recording materials continuously at a print interval that is the same as that for large size recording materials, temperature of an area of a heater through which the recording materials do not pass (i.e., no sheet pass-through area) increases excessively. If temperature of the no sheet pass-through area of the heater increases excessively, a holder for holding the heater or a pressure roller may be damaged by heat. Therefore, the printer equipped with the fixer using the fixing film performs control to increase the print interval in the case of printing small size recording materials continuously compared with the case of printing large size recording materials continuously, so as to prevent temperature of the no sheet pass-through area of the heater from increasing excessively. However, the control of increasing the print interval reduces the number of sheets that can be output per unit time, and therefore it is desired to control the number of sheets that can be output per unit time to be almost the same or just a little smaller than that in the case of large size recording materials. Therefore, as the heater that is used for the above-mentioned fixer, two electrodes are provided to a heater substrate along the longitudinal direction of the heater substrate. Further, it is conceived to use the heater including a heat generation resistive member having a positive temperature coefficient (PTC) disposed between the two electrodes as described in Japanese Patent Application Laid-Open No. H05-019652, for example.
FIG. 15 illustrates an example of the heater. In FIG. 15, the heater includes a heater substrate 214, and electrodes 221 and 222. A feed power connector is connected to areas 221a and 222a. The two electrodes 221 and 222 are disposed along the longitudinal direction of the substrate 214. The heater includes a heat generation resistive member 215 as an electric heat generation member connected between the two electrodes 221 and 222. In addition, FIG. 16 is a circuit diagram electrically illustrating the heater of FIG. 15. As understood with reference to FIG. 16, the heater can be considered to have a structure in which an infinite number of resistors 215r are connected in parallel with each other between the two electrodes 221 and 222 (hereinafter, this type of heater is referred to as a pass-through direction energizing type).
As to the above-mentioned heater, if small size recording materials are driven to pass through the area through which large size recording materials pass (large size pass-through area D) for use in the printer, the no sheet pass-through area F is generated outside the area through which small size recording materials pass (small size pass-through area E). Temperature in the small size pass-through area E hardly rises because heat of the area is removed by the recording material. Therefore, a resistance value of the heat generation resistive member 215 in the small size pass-through area E is hardly increased so that power supply to the heat generation resistive member 215 in the small size pass-through area E is maintained. On the contrary, the resistance value of the heat generation resistive member 215 increases because of the temperature rise in the no sheet pass-through area F. Therefore, the current becomes reluctant to flow so that excessive temperature rise in the no sheet pass-through area F can be suppressed.
However, when the above-mentioned heater was actually incorporated in the fixer and was investigated, it was found that unevenness of heat generation distribution occurred in the heat generation resistive member in the longitudinal direction of the heater substrate despite that no recording material was driven to pass through. The reason of that was revealed to be in the resistance of the electrode. The two electrodes disposed along the longitudinal direction of the heater substrate have high conductivity, but a resistance value thereof is not zero. Therefore, the electrode itself causes a voltage drop due to its own resistance. Therefore, in spite of the state in which no recording material is driven to pass through, heat generation amount on the side close to the area contacting with the feed power connector (left side of the heat generation member of FIG. 10) becomes large while heat generation amount on the side far from the area (right side of the heat generation member of FIG. 10) becomes small. The inventor of the present invention proposes a unit for solving this technical problem in Japanese Patent Application Laid-Open No. 2005-234540.